diff --git a/kfet/config.py b/kfet/config.py
index 8e6e0901..5023e8b0 100644
--- a/kfet/config.py
+++ b/kfet/config.py
@@ -1,6 +1,7 @@
 # -*- coding: utf-8 -*-
 
 from django.core.exceptions import ValidationError
+from django.db import models
 
 from djconfig import config
 
@@ -15,26 +16,51 @@ class KFetConfig(object):
     prefix = 'kfet_'
 
     def __getattr__(self, key):
-        return getattr(config, self.get_dj_key(key))
+        return getattr(config, self._get_dj_key(key))
 
     def list(self):
+        """Get list of kfet app configuration.
+
+        Returns:
+            (key, value) for each configuration entry as list.
+
+        """
+        # prevent circular imports
         from kfet.forms import KFetConfigForm
         return [(field.label, getattr(config, name), )
                 for name, field in KFetConfigForm.base_fields.items()]
 
-    def get_dj_key(self, key):
+    def _get_dj_key(self, key):
         return '{}{}'.format(self.prefix, key)
 
-    def set_many(self, **kwargs):
+    def set(self, **kwargs):
+        """Update configuration value(s).
+
+        Args:
+            **kwargs: Keyword arguments. Keys must be in kfet config.
+                Config entries are updated to given values.
+
+        """
+        # prevent circular imports
         from kfet.forms import KFetConfigForm
+
+        # get old config
         new_cfg = KFetConfigForm().initial
-        new_cfg.update({self.get_dj_key(key): value
-                        for key, value in kwargs.items()})
+        # update to new config
+        for key, value in kwargs.items():
+            dj_key = self._get_dj_key(key)
+            if isinstance(value, models.Model):
+                new_cfg[dj_key] = value.pk
+            else:
+                new_cfg[dj_key] = value
+        # save new config
         cfg_form = KFetConfigForm(new_cfg)
         if cfg_form.is_valid():
             cfg_form.save()
         else:
-            raise ValidationError()
+            raise ValidationError(
+                'Invalid values in kfet_config.set: %(fields)s',
+                params={'fields': list(cfg_form.errors)})
 
 
 kfet_config = KFetConfig()
diff --git a/kfet/tests.py b/kfet/tests.py
deleted file mode 100644
index 5bea7afa..00000000
--- a/kfet/tests.py
+++ /dev/null
@@ -1,5 +0,0 @@
-# -*- coding: utf-8 -*-
-
-from django.test import TestCase
-
-# Écrire les tests ici
diff --git a/kfet/tests/test_config.py b/kfet/tests/test_config.py
new file mode 100644
index 00000000..79781c0d
--- /dev/null
+++ b/kfet/tests/test_config.py
@@ -0,0 +1,49 @@
+# -*- coding: utf-8 -*-
+
+from decimal import Decimal
+
+from django.test import TestCase
+from django.utils import timezone
+
+import djconfig
+
+from gestioncof.models import User
+from kfet.config import kfet_config
+from kfet.models import Account
+
+
+class ConfigTest(TestCase):
+    """Tests suite for kfet configuration."""
+
+    def setUp(self):
+        # load configuration as in djconfig middleware
+        djconfig.reload_maybe()
+
+    def test_get(self):
+        self.assertTrue(hasattr(kfet_config, 'subvention_cof'))
+
+    def test_set_decimal(self):
+        """Test field of decimal type."""
+        subvention_cof = Decimal('10')
+        # IUT
+        kfet_config.set(subvention_cof=subvention_cof)
+        # check
+        self.assertEqual(kfet_config.subvention_cof, subvention_cof)
+
+    def test_set_modelinstance(self):
+        """Test field of model instance type."""
+        user = User.objects.create(username='foo_user')
+        account = Account.objects.create(trigramme='FOO',
+                                         cofprofile=user.profile)
+        # IUT
+        kfet_config.set(addcost_for=account)
+        # check
+        self.assertEqual(kfet_config.addcost_for, account)
+
+    def test_set_duration(self):
+        """Test field of duration type."""
+        cancel_duration = timezone.timedelta(days=2, hours=4)
+        # IUT
+        kfet_config.set(cancel_duration=cancel_duration)
+        # check
+        self.assertEqual(kfet_config.cancel_duration, cancel_duration)
diff --git a/kfet/views.py b/kfet/views.py
index 34e640b3..9238cabd 100644
--- a/kfet/views.py
+++ b/kfet/views.py
@@ -944,8 +944,8 @@ def kpsul_update_addcost(request):
     account = trigramme and Account.objects.get(trigramme=trigramme) or None
     amount = addcost_form.cleaned_data['amount']
 
-    kfet_config.set_many(addcost_for=account and account.pk or None,
-                         addcost_amount=amount)
+    kfet_config.set(addcost_for=account,
+                    addcost_amount=amount)
 
     data = {
         'addcost': {